I. Field of the Invention
This invention relates to fluorine-containing aromatic dicyanate resin compositions capable of being cured at reduced temperatures to form products such as coatings or coatings having low dielectric constants ranging between about 2.8 and 3.1 at 817 MHz, and/or prepregs and laminates having low dielectric constants ranging between about 3.8 and 4.0, high degradation temperatures, thermal stability and flame retardance properties. According to preferred embodiments, this invention relates to the preparation of curable resin compositions for impregnating reinforcing fabrics to form prepregs which are laminated and cured at about 190.degree. to 270.degree. C. in association with conductive metal layers to form circuit boards, the laminates having dielectric constants below about 4.0, glass transition temperatures above about 215.degree. C. and thermal degradation temperatures above 400.degree. C.
II. Description of the Prior Art
It is known, such as from the aforementioned U.S. Pat. No. 4,745,215, that dicyanato diphenyl hexafluorinated alkanes can be impregnated into suitable reinforcing fabrics and heat cured at elevated temperatures above their glass transition temperatures, i.e., about 320.degree. C. for 1 hour. The cured products have excellent properties for high temperature and/or electrical insulation uses, such as in laminated circuit boards. These properties include low dielectric constants, high glass transition temperatures and high thermal degradation temperatures. However the high glass transition temperatures require that the compositions be cured at high temperatures which, in the case of circuit board laminates, requires the use of high temperature lamination presses which is an important disadvantage with respect to manufacturing time and expense.
It is also known to prepare prepregs from dicyanate resins and polyester resins as taught by West German Patent 3,413,547, the use of mixtures of dicyanates including 6F dicyanate, which is hexafluoro-2,2-bis-(4-isocyanato phenyl) propane, being disclosed.
It is known to produce non-fluorinated cyanurate polymers and compositions containing mixtures of such polymers with other polymers to provide compositions having relatively low curing temperatures. Such compositions are burnable unless the added polymer(s) has flame retardant properties and/or unless flame retardency-imparting ingredients are added. Moreover, while the added polymer(s) substantially reduce the normally-higher glass transition temperature and curing temperature of the mixed polymer compositions, they often result in compositions having substantially higher dielectric constants than the normally lower dielectric constant of the cyanurate polymer, per se. Moreover the heat stability and degradation temperature of such mixed polymer compositions generally are reduced to the point that laminates thereof, such as circuit boards, cannot withstand high temperature excursions in the range of 340.degree.-360.degree. C. for even short periods of time (10-15 minutes). Reference is made to U.S. Pat. Nos. 4,477,629; 4,665,154; 4,740,584 and 4,782,116.
Among the resins frequently blended with dicyanate polymers to produce prepreg and reinforced laminate compositions for circuit boards are the lower molecular weight bisphenol A diglycidyl ether epoxy resins, including bromine-substituted resins for imparting some degree of flame resistance as disclosed by U.S. Pat. No. 4,782,116. Such epoxy resins are of relatively low equivalent weight, using non brominated resin for example, so that the epoxy group and possibly the hydroxy group contents are relatively high, i.e., each relatively short repeating unit contains two functional groups, which results in an increase in the dielectric constant of the compositions after curing. Some proposed mixtures contain substantial amounts of the epoxy resin relative to the dicyanate polymer(s), producing an even higher dielectric constant. Also in such compositions the thermal stability or heat resistance of the cured prepregs or laminates is unsatisfactory in high temperature processing applications.